Technical Abstract:
Information is needed on organic phosphorus contribution to the sources and sinks of dissolved phosphorus (PO4'P) in the soil'manure'water system because complexed P forms can be mobilized to alter solution-phase dissolved P concentrations. In samples (n= 107) of dairy manure collected across the northeastern region of the USA, a phytase-hydrolyzable phosphorus (PHP) fraction was found native to ruminant animal manure, in addition to water-extractable P (WEP) that varied considerably between sampling locations (40 to 8630 mg kg-1). A large P fraction was readily hydrolyzable by exogenous phytases and an additional 28.4 ± 9.4 % of total P was released as orthophosphate over and above the level of WEP. Sources of the dissolved P are comprised of complexed inorganic and organic compounds that include phytate (IP6). Counterions such as Na+, Ca2+, Al3+, and Fe3+ and cation to IP6-P mole ratios were found to influence the enzymatic dephosphorylation of organic P. Dephosphorylation of IP6 by Aspergillus ficuum (Reichardt) Henn. phytase EC 3.1.3.8 decreased by 50 ± 3.6 and 40 ± 4% at pH 4.5 and 6, respectively, as Ca2+ levels increased and Ca2+ to IP6-P mole ratios reached 6:6. Polyanionic IP6 has a high affinity for Al3+ and Fe3+ and dephosphorylation was reduced by an average 27 and 32% at a cation to IP6-P mole ratio of 1:6 for Al3+ and Fe3+, respectively, while reaching more than 99% at a mole ratio of 6:6. Therefore, the enzyme-hydrolyzable P fraction in manure can be significantly underestimated because complexed PHP was not susceptible to dephosphorylation by the phytases. By the same token, reduction in mobilization and biological dephosphorylation of inositol phosphates may be achieved with a judicious choice of the type and quantity of polyvalent counterions and the control of exogenous enzymatic activity. A modified fractionation method was developed to characterize water-desorbable inorganic and organic P coordinatively associated with polyvalent cations. The fractionation conditions are mild and its simplicity may increase the practicality of widespread composition characterization and P bioavailability measurements in animal manure and the development of mitigation practices to reduce mobilization of bioactive organic P.